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Modeling Soybean Cultivar Development Rates, Using Field Data from the Mississippi Valley

USDA-ARS, Bldg. 007, Rm. 008, BARC-West, Beltsville, MD 20705

Yakov A. Pachepsky^*

Duke Univ., Phytotron and USDA-ARS, Remote Sensing and Modeling Lab., Bldg. 007, Rm. 008, BARC-West, Beltsville, MD 20705

Mary C. Acock and Vangimalla R. Reddy

USDA-ARS, Remote Sensing and Modeling Lab., Bldg. 007, Rm. 008, BARC-West, Beltsville, MD 20705

Frank D. Whisler

Dep. of Plant and Soil Sciences, Mississippi State Univ., Mississippi State, MS 39762

^* Corresponding author (ypachepsky{at}assr.arsusda.gov).

To predict how crop yields of soybean [Glycine max (L.) Merr.] respond to management practices, we need a reliable understanding of how environmental factors affect ontogeny of the many soybean cultivars grown. The objective of this work was to use field observations to develop equations relating reproductive development to environmental variables for several cultivars grown commercially in the southern USA. We used data on five soybean cultivars, recorded during farm testing of the soybean crop model GLYCIM in 1993, 1994, and 1995. Crop development stages were determined by averaging the Fehr and Caviness development stages of individual plants. We inspected the data and found that it could be fitted using a piecewise linear regression with alternating horizontal and sloping segments. The progress in crop reproductive development stages was expressed as a function of time after emergence for the period before flowering, and as a function of thermal time for the period after flowering. Horizontal segments that occurred near R2, R5, and R6 were interpreted as developmental plateaus (i.e., a period when the crop remains at a given stage). The length of the R2 plateau was a function of date of emergence, and the length of the R6 plateau was a linear function of soil surface water balance. In our dataset, there was no effect of soil properties, irrigation, or plant population density on reproductive development. The technique developed in this study will facilitate the application of crop simulation models on farms by providing a way to develop a set of parameters for reproductive development for any cultivar being grown.

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